The present invention relates to a bit error ratio detector, and more particularly to a bit error ratio detector for digital radio communication using error correction codes.
In a digital radio communication system, the quality of the working transmission line is monitored and, when the quality of the line degrades, either the line is switched to a standby line or an alarm indication signal (AIS) is issued. A technique used for this monitoring of line quality is embodied in bit error ratio detector which assess line quality by calculating the bit error ratio (BER). Some of these bit error ratio detectors use a syndrome. A bit error ratio detector using a syndrome subjects coded data from the transmitting side, error correction-coded block by block, to error correcting operation, and outputs the syndrome. The BER detector detects the presence or absence of any code error in each block responsive to the syndrome and calculates a code error ratio over a period of a plurality of blocks. By converting this code error ratio into a BER, the quality of the line is assessed.
Meanwhile, when t-multiple error correction code is used, if this block length is reduced, the redundancy will generally increase, resulting in a decline in the efficiency of frequency utilization in the digital radio communication system. It is therefore desirable for increasing the efficiency of line utilization to extend the block length so as to achieve a proper level of redundancy. However, a extended block length means a correspondingly higher probability of syndrome occurrence, further resulting in a correspondingly higher probability of the occurrence of error pulses generated by the syndrome. The probability of error pulse occurrence rises with an increase in BER, eventually reaching 100%. As a consequence, where the bit error ratio so much deteriorates as to surpass, for instance 10.sup.-3, the probability of error pulse occurrence will no longer significantly vary with a change in BER. This means that a BER higher than a certain level makes it impossible for the quality of the line to be accurately assessed.